The hydrogenperfluoroalkanes are known compounds which can be used, for example, as inert fluids in the electronic field, as heat transfer fluids or as intermediate products for the synthesis of other fluorinated compounds, for example for the preparation of perfluoroalkyl bromides R.sub.F Br, of which some (especially C.sub.8 F.sub.17 Br) have found an application in medicine as radiopaques (X-ray contrast agents) or as oxygen carriers in blood substitutes.
Among the numerous methods known to prepare hydrogenperfluoroalkanes, the most attractive, because they start from perfluoroalkyl iodides R.sub.F I which are industrial products, consist in reacting a compound R.sub.F I with alcoholic potash at a temperature of 100.degree. to 130.degree. C. (see J. Banus et al, J. Chem. Soc. 1951, 60 and particularly R. N. Haszeldine, J. Chem. Soc. 1953, 3761-8) or with hydrogen under pressure in the presence of Raney nickel (R. N. Haszeldine, ibid. and French Patent 2,116,524).
Although it appears to give the best yields (&gt;80%), this last method has the disadvantage, on the one hand, of being carried out in an autoclave under a pressure of several tens of bars of hydrogen and, on the other hand, of not leading to a total selectivity because doubling products R.sub.F --R.sub.F are formed which contaminate the hydrogenperfluoroalkanes obtained.
The first method (treatment of the R.sub.F I by alcoholic potash) does not have these disadvantages, but unfortunately leads to a yield of compound R.sub.F H (27 to 51%) which is lower as the number of carbon atoms of the radical R.sub.F is increased (51% for C.sub.3 F.sub.7 H and 27% for C.sub.8 F.sub.17 H). In addition, the formation of tars is seen which prevent the filtration of the reaction mixture and render impossible the recovery of the potassium iodide by-product. It is the same when ethanol is replaced by isopropanol, used in U.S. Pat. No. 4,618,731 in relation to the purification of 2-perfluoroalkylethanols containing the compounds R.sub.F CH.sub.2 CH.sub.2 I and R.sub.F I as impurities.